The mitotic spindle is correctly positioned within the dividing cell is a critical but poorly understood aspect of mitosis in S. cerevisiae. Spindle positioning is essential for cell division in many organisms and is a key regulated step for the generation of asymmetric cell divisions of specific cell types during development. Spindle positioning is thought to be mediated by interactions between astral microtubules and asymmetrically localized factors at the cell cortex. The specific molecular basis for spindle positioning is not known, but in several organisms it involves interactions between the actin and tubulin cytoskeletons. When spindle positioning fails, yeast cells delay in the initiation of cytokinesis. This cell cycle delay has the characteristics of a checkpoint mechanism because it can be bypassed by specific mutations. Thus yeast has mechanisms to "sense" defects in spindle position and correct them (the cytokinesis checkpoint). The goal of this proposal is to define the molecular mechanism of spindle positioning and characterize the signaling processes that link spindle position to other cell cycle events. Recent work form this laboratory has identified key components of this system and initiated the characterization of their biochemical and in vivo functions. Because of the central role of spindle positioning in development, this work may impact on the understanding of inherited and acquired developmental diseases.